Development and the After Process

After exposure to light a small number of silver metal atoms are formed (a latent image). These atoms act as a catalyst, sensitizing the surrounding halide salt so that on additon of a develpoer (a reducing agent) this is reduced, producing a black silver metal in the area exposed to light. The reducing agent is usually hydroquinone which reacts as follows;

This reaction takes place at a relstively high pH (alkaline conditions) which means that when the image has developed to the desired degree it can be halted by dipping the film in acid, most commonly a solution containing acetic acid. This process is known as stopping. It also prevents the production of fog. The action of the acid is very quick so it only needs seconds for the process of development to be sucessfully halted.

Here is a table of commonly used developers;

Fixing

Fixing makes the image permanent because as it is a large amount of unexposed and unreacted silver halide remains on the photography film, in the emulsion. If the halide is not removed the image will continue to darken. Complex formation is used to remove unexposed AgX (silver halide). Thiosulphate is used to complex the silver halide, producing the complex [Ag(S₂O₃^2-]₂^3- which is washed away to leave only the dark silver metal image. This image is a negative, through which light is shone to produce the final photgraph in the same process as was used to produce the negative. Soduim Thiosulphate has been used since 1839 for this purpose.

The reaction is as follows;

AgBr + 3Na₂S₂O₃ →  Na₅[AgS₂O₃] + NaBr

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